This invention relates to a sputter deposition method which is especially suitable for use in the manufacture of organic devices, especially ones that have a conjugated polymer as a light-emitting layer.
One type of organic device is described in U.S. Pat. No. 5,247,190, the contents of which are incorporated herein by reference. The basic structure of this device is a light-emitting polymer film (for instance a film of a poly(p-phenylenevinylene)xe2x80x94xe2x80x9cPPVxe2x80x9d) sandwiched between two electrodes, one of which injects electrons and the other of which injects holes. It is believed that the electrons and holes excite the polymer film, emitting photons. These devices have potential as flat panel displays.
In more detail, such an organic light-emitting device (xe2x80x9cOLEDxe2x80x9d) typically comprises an anode for injecting the positive charge carriers, a cathode for injecting the negative charge carriers and, sandwiched between the electrodes, at least one electroluminescent organic layer. The anode is typically a layer of indium-tin oxide (xe2x80x9cITOxe2x80x9d) which is deposited on a glass substrate. The organic layer(s) are then deposited on the anode and the cathode is then deposited on the organic layer(s) by, for example, evaporating or sputtering. The device is then packaged for protection.
The organic layer is typically formed by conversion from a precursor form or by spin coating a soluble organic material. Organic layers formed by conversion often contain by-products of the conversion reaction (e.g. acids) which can harm the adjacent electrodes. However, soluble organic layers are more sensitive and prone to damage during later process steps, for instance when an adjacent electrode is deposited.
A cathode of high quality is of great importance to achieve overall high performance in OLEDs, judged on criteria such as power efficiency, low drive voltage, shelf life, operating life and stability in stringent environmental conditions such as high temperature and/or high humidity. The criteria for the quality of the cathode are in particular but not exclusively the work function, corrosion resistance, morphology and barrier properties, adhesion to the polymer and sheet resistance.
Metallic cathode layers for OLEDs are most commonly deposited by simple thermal evaporation of the cathode material in vacuum. Similarly, cathode layers consisting of a metal alloy can be deposited by thermal evaporation from two or more sources containing the alloy constituents and by choosing appropriate relative depositing rates to achieve the desired relative alloy composition.
However, simple thermal evaporation of metals onto OLEDs to form a cathode layer can result in poor adhesion between the cathode and the top organic layer and, very often, the morphology of the evaporated layer is polycrystalline with large average grain size such that there is an increased likelihood of pinholes providing potential pathways for the ingress of ambient gases such as oxygen and moisture into the device. Poor adhesion and large grain-size polycrystalline morphology can severely deteriorate the OLED performance, in particular environmental stability (device shelf-life and operating life, corrosion of the cathode).
The same issues (adhesion, morphology) apply to the case in which an OLED is built up from the cathode, i.e. when the cathode is deposited on the substrate with the subsequent deposition of the organic layer(s) and as the final step deposition of the anode on top of the top organic layer.
If possible, sputtering might be a preferred method of depositing the cathode because sputtered films tend to have better adhesion, better density and less susceptibility to pin-hole defects, all of which are important to the performance of the device. Sputtering is also desirable for economic reasons and because of the higher throughput it should allow. However, it has been found to be very difficult to sputter on to organic layers. Some approaches for sputtering on to organic layers have been successful but, in general, sputtering processes can cause significant damage to underlying organic layers. This is especially important if sputtering directly on to delicate organic materials such as soluble polymers is contemplated. Therefore, sputtering has not found widespread acceptance. Instead evaporation has often been preferred as a method of depositing the cathode layers, despite the fact that it tends to produce poorer cathode layers, because it results in less damage to the underlying organic layer.
One aim of the present invention is to provide a method of sputter depositing which results in less damage.
According to the present invention from one aspect there is provided a method of sputter deposition on to an organic material, wherein the discharge gas of the sputtering operation is a gas having a spectrum of light emission of a lower energy than that of argon.
Argon is generally used as the discharge gas in sputtering processes. According to the present invention the preferred discharge gas is neon, or a mixture containing neon. The discharge gas preferably has a molecular weight less than that of argon.
Preferably the method is a method of sputter depositing material on to a substrate of an organic material in a vacuum chamber, comprising the steps of: introducing into the chamber a discharge gas having a spectrum of light emission of a lower energy than that of argon; and sputter depositing material on to the substrate.
The organic material may be a soluble material and/or a solvent-based material, with the solvent preferably being water. The material may be a polymeric material. The material may be a luminescent material. The material is preferably a conjugated or partially conjugated material, most preferably a conjugated polymer material. The material preferably comprises an electroluminescent polymer, such as PPV, poly(2-methoxy-5(2xe2x80x2-ethyl)hexyloxyphenylene-vinylene) (xe2x80x9cMEH-PPVxe2x80x9d), a PPV-derivative (e.g. a di-alkoxy derivative), polyfluorenes, polyparaphenylenes, polythiophenes, etc. or copolymers thereof and including substituted and/or unsubstituted versions thereof. The material may comprise a luminescent organometallic polymer. The material may comprise a small molecule luminescent material (see U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference) such as tris(8-hydroxyquineleto)aluminium (Alq3).
The organic material is preferably deposited prior to the sputtering operation, and is most preferably deposited from solution. It could be deposited by (for instance) spin-coating, dip-coating, blade-coating, meniscus-coating or self-assembly.
The organic material preferably takes the form of a layer. The thickness of the layer is preferably in at least the range from 2 to 200 nm and most preferably around 100 nm.
The method preferably involves sputter depositing material directly on to the organic material from a sputter target or sputter cathode. The target could comprise a metal, a metal alloy or a metal oxide. The target may be a low work function material (with a work function less than 3.5 eV or 3.0 eV) or a high work function material (with a work function greater than 4.0 eV or 4.5 eV). The target could be a powder target. Specific examples of target materials (and therefore sputtered materials) include Al, Zr, Mg, Si, Sb, Sn, Zn, Mn, Ti, Cu, Co, W, Pb, In or Ag or alloys thereof and/or low work function elements such as Li, Ba, Ca, Ce, Cs, Eu, Rb, K, Sm, Na, Sm, Sr, Tb or Yb. A typical such alloy would, for instance, be a commercially available Al95%/Li2.5%/Cu1.5%/Mg1% alloy.
The method may also comprise a step of conditioning the sputter target prior to the step of sputter deposition. The conditioning step suitably involves sputtering material from the target, suitably to remove impurities (such as oxygen) from the target. During the conditioning step the organic material is suitably out of range of the target, or shielded from it. The conditioning step is preferably carried out using a discharge gas having a spectrum of light emission of at least that of argon. Preferably the discharge gas for the conditioning step is argon.
The sputtering and/or the pre-conditioning steps may be carried out in the presence of further gasses in addition to the discharge gasses. One purpose of the further gasses is coolingxe2x80x94a relatively light gas such as helium may be used for this.
The sputtering may be by any suitable sputter process. The process could be a DC or an RF sputtering process. The alloy morphology achieved specifically by DC magnetron sputter deposition can act to minimise, for example, segregation and diffusion effects within the cathode alloy after the deposition. The process could be a reactive sputtering process or a non-reactive process. The process could be a magnetron sputtering process or not. For reactive sputtering the sputter deposition step may be carried out in the presence of a reactive gas, for example oxygen and/or nitrogen. The method may then provide for the deposition of inorganic oxides and/or nitrides.
The method preferably provides for the deposition of a layer on to and in contact with the organic material. The layer may be an electrode layer. The layer preferably comprises the material of the sputter target/cathode. The layer is preferably substantially free from pin-hole defects. The layer preferably comprises aluminium and/or calcium and/or lithium. The layer is preferably well adhered to the organic layer. The layer is preferably of compact morphology with low average grain size and good adhesion to the organic material. Good adhesion between the cathode and the adjacent layer minimises delamination and the ingress of, for example, oxygen, moisture, solvents or other low molecular weight compounds at/along said interface. Also, the compact morphology of the cathode metal layer can help reduce diffusion of ambient species such as oxygen, moisture, solvents or other low molecular weight compounds into the organic material through the cathode layer itself.
The sputtering operation may be carried out directly on to the organic material or on to an intermediate layer on the organic material. In the latter case the intermediate layer is preferably of an electrically conductive material, for instance consisting principally of one or more of the metals listed above. The intermediate layer may be deposited by evaporation.
A further layer of electrically conductive material may be deposited over a sputtered layer formed as set out above, whether the sputtered layer is deposited directly on to the organic material or not. The further layer could be deposited by evaporation.
Preferably the method also comprises the step of depositing the organic material from a solvent (for instance by spin coating, dip-coating, blade-coating, meniscus-coating or self-assembly) prior to the sputter deposition step.
After the sputter deposition is complete the organic layer is preferably less damaged than would be expected if argon were used as the sputtering gas, and most preferably substantially undamaged.
According to the present invention from another aspect there is provided an organic light-emitting device comprising an electrode layer deposited by sputter deposition according to the method of the present invention. In particular, such an organic light-emitting device may comprise:
a first electrode layer for injecting charge carriers of one polarity;
a second electrode layer for injecting charge carriers of the opposite polarity and deposited by a sputter deposition process in which the discharge gas has a spectrum of light emission of a lower energy than that of argon; and
an organic light emissive layer located between the electrode layers.